Implement Git Hooks to improve the engineer experience

[stuartc]

Description

This PR adds automatic code formatting on commit using elixir_git_hooks.

When you commit code, git hooks now automatically format your staged files:

• Elixir/Phoenix files (.ex, .exs, .heex) with mix format
• JavaScript/TypeScript files (.js, .ts, .tsx, .jsx) with prettier
• Markdown files (.md) with prettier

The hooks format the files and re-stage them, so your commit always contains properly formatted code. If the formatter removes your only changes (like a blank line that violated formatting rules), the commit is blocked with a clear message.

Re #269

Validation steps

1. Make a change to an Elixir file with bad formatting (e.g., remove spaces around operators)
2. Stage and commit the file
3. Verify the pre-commit hook formats it automatically
4. Check the committed file is properly formatted
5. Try committing a file where the only change is a formatting error that gets auto-fixed
6. Verify the commit is blocked with "No changes to commit after formatting"

Additional notes for the reviewer

The hooks run on pre-commit (not pre-push as suggested in #269). This catches formatting issues earlier in the workflow. The hooks are configured in config/dev.exs and only run in development mode.

The git_hooks package auto-installs hooks when you run mix compile, so developers don't need to manually set anything up.

AI Usage

• Code generation (copilot but not intellisense)
• Strategy / design
• Optimisation / refactoring
• Learning or fact checking
• Translation / spellchecking / doc gen
• Other
• I have not used AI

Pre-submission checklist

• I have performed a self-review of my code.
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR